Chapter 33
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Synthesis and Photochemistry of a 2,6Dialkoxyanthracene-Containing, Side-ChainSubstituted Liquid-Crystalline Polymer 1
2
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1
David Creed , Charles E. Hoyle , Anselm C. Griffin , Ying Liu , and Surapol Pankasem 2
1
2
Department of Chemistry and Biochemistry and Department of Polymer Science, University of Southern Mississippi, Hattiesburg, MS 39406 The synthesis and several aspects of the photophysical and photochemical behavior of a methacrylate polymer substituted, via a flexible methylene chain, with a 2,6-dialkoxyanthracene chromophore are reported. Polarized light microscopy and DSC indicate the polymer is liquid crystalline. The UV-Vis and fluorescence spectra of the polymer indicate both ground and excited state aggregation of the chromophores as evidenced by perturbations of these spectra relative to those of a simple model compound. Chromophore association in the pound state seems to occur even in a good solvent, dichloromethane. Chromophore aggregation effects are enhanced in films of the polymer, particularly after heating or when poor solvents are added to a solution of the polymer in dichloromethane. Steady state and time-resolved fluorescence experiments indicate significant self-quenching of the excited anthracene chromophore even in highly diluted solutions in dichloromethane and the presence of a long-lived (τ = 30.6ns) intramolecular excimer or excited aggregate and a short -lived(τ = 2.0ns) anthracene singlet state. At higher concentrations an intermolecular excimer or excited aggregate is observed. Fluorescence spectra of films indicate the presence of weakly emissive excimers or excited aggregates. Two types of 4 + 4 photocycloaddition products are most likely formed upon UV or visible light irradiation. Photooxidation products are also formed when irradiation is carried out in the presence of oxygen.
The photochemistry of liquid crystalline (LC) polymers is of fundamental interest because LC states combine some of the characteristics of both highly ordered crystals and disordered but mobile liquids. Crystals are 3-dimensionally ordered but the molecules in a crystal can only undergo relatively small movements about their 0097-6156/95/0614-0504$12.00/0 © 1995 American Chemical Society
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33. CREED ET AL.
Synthesis & Photochemistry of Liquid-Crystalline Polymer
equilibrium positions in the lattice. In an isotropic liquid, the molecules can move much more freely but there is no long-range order. LC materials combine 1- or 2dimensional order with some degree of fluidity. LC materials also have a variety of applications or potential applications in which they are inadvertently or deliberately subjected to ultra-violet and visible irradiation with frequently unpredictable effects on their structures and properties. Irradiation may be deliberately used to modify structural properties or in photoimaging applications of a LC polymer. Alternatively, a structural polymer may be degraded by sunlight, or a polymer designed for non-linear optical (NLO) applications may be sensitive to the radiation source used in the NLO-based device. All of these considerations have prompted us to begin a comprehensive study of the influence of LC structure on the photochemistry of polymers. We have recently reported several aspects of the photochemical and photophysical properties of LC polymers (i), including main- (2) and side-chain (3) substituted poly(aryl cinnamates) and main-chain poly(stilbene-bis-carboxylates) (4). In all of these materials, a conjugated chromophore, either aryl cinnamate or stilbene bis-carboxylate, serves as the rigid mesogenic group in the polymer. Among the most interesting effects observed to date is the formation of chromophore aggregates which results in the observation of perturbed UV-Vis absorption spectra of all these materials, wavelength dependent photochemistry (5) of the poly (aryl cinnamates), presumably as a consequence of the chromophore heterogeneity that accompanies incomplete or cooperative chromophore aggregation, and hyperchromism upon initial irradiation of fluid LC phases of these materials (/J), that is attributed to disruption of chromophore aggregates as photoproducts begin to form and affect phase behavior. The extent of aggregation is dependent on the LC mesophase. Smectic mesophases, which have 2-dimensional ordering with layering of the mesogenic (chromophoric) groups, show more dramatic spectral perturbations (3) than do nematic mesophases, which have only 1-dimensional ordering of the mesogens. We suspect that aggregation driven shifts of absorption spectra to longer wavelengths may contribute to the environmental degradation of many polymers. In this paper we report the synthesis and some preliminary observations of the photophysics and photochemistry of a side-chain substituted LC polymer, 4, containing a different, photochemically reactive 2,6-dialkoxyanthracene chromophore. Our studies of this type of material were motivated by the well known reversible 4 + 4 photocycloaddition chemistry of anthracene (6), the ability of anthracene to fluoresce, which is an advantage in studying the fate of excitation energy absorbed by the polymer, and the many previous reports of the photophysics and photochemistry of anthracenes and anthracene containing polymers. Experimental Materials and Characterization. All starting materials were obtained from Aldrich. Methacryloyl chloride was distilled under reduced pressure before use. 2,2'-azobis(isobutyronitrile) was recrystallizedfrommethanol and dried in vacuo. Dimethylformamide (DMF) and benzene were dried over CaH and distilled (DMF 2
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MICROELECTRONICS TECHNOLOGY
0(CH )9CH3 2
HO(CH ) 0 2
6
0(CH )9CH3 2
HO(CH ) 0 2
6
I CH =CH(CH )C0C1 2
3
0(CH )9CH3 2
CH ==CH(CH )COO(CH ) 0 2
3
2 6
3 I AIBN *
